True that it's harder to find filters in Cokin A-size, but the only square filter you really need is the ND grad, which is quite widely available for around $20. I use a 10-stop ND and a polarizer on my M43 kit, and both are screw-in models (B+W #110 ND 3.0 and B+W Kaesemann MRC Polarizer) in 46mm thread. This thread size fits all my lenses (although sometimes I use step-up or step-down rings, like the one you can see in the picture of my camera with Cokin A filter).

I could just imagine it being a bit awkward adjuting a polarizer behing a set of square filters. I might be wrong. Also although I'm awake there are screw in 10 stop filters I figure if I'm going to have a filter holder on my camera anyway it might be easier to just have a slot in one.

Dreeke
wrote:

Does it have to be on the front?

Here are two pictures of my cokin A filerholder with a round polarizer at the back (at the lens side) and a gradual grey in front.

I haven't heard great stuff about the quality of the cokin polarizers. I might be misinformed but having an extra filter ring on the front for normal filters does seem like it might come in handy to me. I'll have a much larger range of what I can choose to use.

Well, if you're going to have a slot-in polarizer then it'll probably be a Cokin. In my case, I put on the polarizer first and adjust it for the light. Then I put the square Cokin grad filter over the top using Blutack (see my Blutack photo earlier). Then you don't need a holder at all.